Dendritic cells (DC) play a pivotal role in the initiation and modulation of adaptive immunity through the release of polarizing cytokines, as well as the processing and presentation of captured antigen (Ag) to prime or recall specific T cell responses. Immunotherapy strategies, including vaccination, aiming at modulating immune responses, have been focusing on the delivery of Ag to these professional antigen-presenting cells (APC). Vaccination is usually achieved by the in vivo delivery of antigenic entities (DNA, peptides or proteins), that are, due to their low immunological activities, generally formulated in adjuvanted delivery systems.
Several delivery systems have been investigated during the past years, where different adjuvants, based in particular on Ag particulation, have been tested for their efficacy in enhancing antigen delivery and thus activating the immune system. For instance, the adjuvant effect of liposomes in antigen-liposomes formulations has been investigated (see e.g.: Tanaka et al, “Liposomes with Differential Lipid Components Exert Differential Adjuvanticity in Antigen-Liposome Conjugates via Differential Recognition by Macrophages”, Bioconjugate Chem., Vol. 15 (1), 2004, pp. 35-40). More recently, also gas-filled microvesicles have been proposed as immune adjuvants and vaccine carriers, as disclosed in Chinese Patent CN 10054664. Gas-filled microvesicles are generally known for their use as contrast agents, particularly for ultrasound imaging. They typically include suspensions of gas bubbles having a diameter of a few microns dispersed in an aqueous medium and comprise suitable materials forming a stabilizing envelope for containing the gas.
CN 10054664 discloses in particular the preparation of gas-filled microvesicles where an antigen is either encapsulated inside the microvesicle or adhered to the surface thereto (by static electrical adsorption). The microvesicles are then administered topically and ultrasonic waves are applied locally, to destroy the microvesicles and release the antigen. According to said patent, contrary to the positive effects observed with combined use of gas-filled microvesicles and ultrasound irradiation, the sole administration of microvesicles with an antigen adhered thereto is substantially ineffective (comparable to a blank control).
International Patent Application WO 2012/085072 discloses gas-filled microvesicles where the antigen is covalently bound to a component of the microvesicle envelope, in particular a phospholipid, these microvesicles providing a remarkable adjuvant effect in the substantial absence of any ultrasound irradiation.
The Applicant has now found that by suitably modifying the formulation of the stabilizing envelope of the gas-filled microvesicles the adjuvant effect of the microvesicle may be further increased. In particular, the Applicant has observed that advantageous results in immunomodulating treatments can be achieved when the molar amount of fatty acids in the stabilizing envelope of the microvesicles is higher than the molar amount of phospholipids; further advantages can be achieved when the antigen is directly linked to a phospholipid.